1. Field of the Invention
The present invention relates to an orthogonal frequency division multiplexing (OFDM) receiver for receiving and demodulating an OFDM signal and its automatic gain controller circuit.
2. Description of Related Art
In recent years, a modulation method called an orthogonal frequency division multiplexing method (hereinafter referred to as an OFDM method) is known as a method of modulating digital data. With this OFDM modulation method, a number of orthogonal subcarriers is provided in a transmission band, and digital data is assigned to the amplitude and phase of each subcarrier by phase shift keying (PSK) or quadrature amplitude modulation (QAM) to modulate the digital data.
Since the OFDM method divides the transmission band by a number of subcarriers, the band per one subcarrier becomes narrower and a modulation speed lowers. However, it has a feature such that a total transmission speed is not different from that of a modulation method of current related art. Since the OFDM method transmits a number of subcarriers in parallel, a symbol speed becomes lowers, thereby enabling to shorten a relative time length of a multipath with respect to a time length of a symbol. It has therefore another feature of less susceptibility to multipath interference. Still another feature is such that since the OFDM method assigns data to a plurality of subcarriers, a transmission/reception circuit can be realized by using an Inverse Fast Fourier Transform (IFFT) calculation circuit for inverse Fourier transform during modulation and a Fast Fourier Transform (FFT) calculation circuit for Fourier transform during demodulation.
The OFDM method having these features are often applied to terrestrial digital broadcasting which is strongly influenced by multipath interference. For terrestrial digital broadcasting adopting the OFDM method, there are standards such as Digital Video Broadcasting-Terrestrial (DVB-T) and Integrated Services Digital Broadcasting-Terrestrial Sound Broadcasting (ISDB-TSB) (e.g., refer to “Terrestrial Digital Sound Broadcasting Receiver Standards (Desired Specification) ARIB STD-B30, ver. 1.1”, Association of Radio Industries and Businesses, executed on May 31, 2001 and revised to 1.1 on Mar. 28, 2001, and “Transmission Method for Terrestrial Digital Sound Broadcasting ARIB STD-B29, ver. 1.1”, Association of Radio Industries and Businesses, executed on May 31, 2001 and revised to 1.1 on Mar. 28, 2001.
In an OFDM receiver of related art, a tuner unit connected to an antenna converts an OFDM signal of a desired reception channel into an intermediate frequency (IF) signal which is then supplied to an intermediate frequency amplifier unit. The IF signal amplified by the intermediate frequency amplifier unit is converted into a digital signal by an A/D converter circuit, and the digital signal is supplied to a baseband processing unit. By using a carrier signal having a predetermined frequency (carrier frequency), the baseband processing unit orthogonally demodulates the digitized IF signal to obtain an OFDM time domain signal of the baseband. Only the effective symbol period of the OFDM time domain signal is subjected to FFT to obtain an OFDM frequency domain signal. The OFDM frequency domain signal is subjected to differential demodulation of DQPSK or synchronous demodulation of QPSK, 16 QAM or 64 QAM to obtain a demodulated output of the OFDM signal of the reception channel.
The baseband processing unit has therein an automatic gain controller (AGC) circuit which maintains constant the level of the intermediate frequency signal to be supplied to the A/D converter circuit by controlling the gain of a voltage controlled variable gain amplifier constituting the intermediate frequency amplifier unit.
For example, as shown in FIG. 8, an automatic gain controller circuit 510 of an OFDM receiver 500 of related art includes an absolute value (ABS) circuit 511, a subtractor circuit 512, a sign judgment (SGN) circuit 513, a low-pass filter 514, a rounding process (RIND) circuit 515, an accumulator circuit 516.
In this automatic gain controller circuit 510, the ABS circuit 511 obtains an absolute value of the signal level value of the intermediate frequency signal by removing the sign of the intermediate frequency signal digitized by and supplied from the A/D converter circuit 504. The subtractor circuit 512 subtracts a fixed target value from the signal level value of the intermediate frequency signal changed into the absolute signal level by the ABS circuit 511 to detect a signed difference value. The SGN circuit 513 judges the sign of the signed difference value detected by the subtractor circuit 512, and supplies a 1-bit sign signal corresponding to the sign of the difference value to the accumulator circuit 516 via the low-pass filter 514 and rounding process (RND) circuit 515. The accumulator circuit 516 outputs an accumulation output as an automatic gain controller (AGC) signal.
The AGC signal output from the automatic gain controller circuit 510 is supplied to a pulse density modulation (PDM) circuit 520 which outputs a PDM signal of a rectangular wave whose density changes with an amplitude of the AGC signal. The PDM signal is fed back via a low-pass filter 525 to a control terminal of a voltage controlled variable gain amplifier 503A constituting an intermediate frequency amplifier unit 503.
As described above, in the automatic gain controller circuit 510 of the OFDM receiver 500, the fixed target value is used as one of input parameters, and subtracted from the absolute value of the signal after A/D conversion. The sign of the resultant value is converted into a 1-bit signal. This signal is passed through the low-pass filter 514 and accumulated in the accumulator circuit 516 to obtain and output the AGC signal.
Input signals to the A/D converter circuit 504 of the OFDM receiver 500 have a property of a Gaussian distribution in an additive white Gaussian noise (AWGN) channel, as shown in FIGS. 9A and 9B. If a distribution of input signals is estimated and a fixed target value is used, a clip amount of the input signal can be adjusted. Adjusting noises by clipping and adjusting noises by quantization are properly balanced so that a quantity of noises mixed in the A/D converter circuit 504 can be minimized.